The present invention relates to a scanning optical system in which a plurality of laser beams scan on a plurality of surfaces to be scanned, respectively.
An apparatus which is capable of performing color printing, such as a color laser printer, a color photo-copying apparatus or the like, is generally known. For example, in the color laser printer, a scanning optical system capable of emitting a plurality of laser beams to scan on a plurality of photoconductive drums is employed. By using this type of scanning optical system, latent images are formed on the plurality of photoconductive drums corresponding to a plurality of color components, respectively.
This type of scanning optical system is provided with a polygonal mirror which rotates about its rotational axis at constant angular speed to deflect laser beams emitted by light sources, and imaging optical systems which converge the deflected laser beams to form beam spots scanning on the photoconductive drums at constant speed, respectively. Thus, a plurality of beam spots formed on a plurality of surfaces to be scanned move along scanning lines, respectively. The extending direction of the scanning line will be referred to as a main scanning direction.
Since each of the photoconductive drums is rotated while the beam spot scans along the scanning line, a plurality of scanning lines, which are evenly spaced in an auxiliary scanning direction perpendicular to the main scanning direction, are formed on each of the photoconductive surfaces. Further, the beam spot is modulated according to image data while such a scanning operation is performed. Therefore, a two dimensional latent image is formed on each of the photoconductive surfaces.
By forming latent images respectively corresponding to the color components on the photoconductive drums, the plurality of color image components are printed, in an overlaid fashion, on the same sheet so that a color image is formed.
To scan along the scanning line at constant speed, the imaging optical system is designed to have fxcex8 characteristics. In addition, for each of the color components, the imaging optical system having the same configuration is used.
With this configuration, it becomes possible that all the laser beams scan on the respective photoconductive surfaces at the same constant speed. In this case, if timings of the modulation for all the color components are synchronized, a dot for each of the color components is formed at the same position in the printed color image. That is, theoretically, the occurrence of the color drift in the printed image can be prevented.
However, there may be a case where speed variations of the beam spot in the main scanning direction occur due to a positional error of the imaging optical system. In such a case, even though the laser beam is modulated (i.e., the laser beam is turned to ON or OFF) at a constant time interval, a space between adjacent dots which are formed along the scanning line varies with its position. As a result, densely dotted portions and sparsely dotted portions appear along the scanning line.
In this case, it is possible to even up a writing start position and a writing end position for each color component by adjusting timings at which the modulation is started and ended for each color component. However, it is not possible to even up centers of the scanning lines for respective color components because the speed variation of the beam spots in the main scanning direction for the color components are different from each other. That is, for example, if 199 dots are included in one scanning line, the 100th dot is not formed at the center of the scanning line.
Therefore, in this case, a dot which is to be formed at the center of the scanning line is actually formed at a position shifted from the center of the scanning line. In addition, positions of the dots of the color components are different from each other in the scanning line. If a printing operation is performed under such situations, the color drift occurs at a central portion of each of the scanning lines. As a result, in the printed color image, the color drift appears within a rectangular area extending in a vertical direction (i.e., the auxiliary scanning direction).
Therefore a scanning optical system, which is configured to prevent occurrence of the color drift at the rectangular area (i.e., the central portion) of the printed color image, is desired.
The present invention is advantageous in that it provides a scanning optical system which is capable of adjusting its fxcex8 characteristics so that a beam spot which should be formed at a center of a scanning line is actually formed at the center of the scanning line.
According to an aspect of the invention, there is provided a scanning optical system for emitting a plurality of laser beams scanning in a main scanning direction. The scanning optical system is provided with a deflector that deflects a plurality of laser beams to scan in the main scanning direction, and an imaging optical system that converges the plurality of laser beams deflected by the deflector to form a plurality of beam spots on surfaces to be scanned, respectively, the plurality of beam spots scanning on the surfaces to be scanned in the main scanning direction, respectively.
Further, the imaging optical system has at least one lens whose position is changeable in a plane including an optical reference axis thereof and parallel with the main scanning direction.
In the above configuration, the imaging optical system satisfies a condition:
0.05 less than |f/fL| less than 0.5
where, f represents a focal length of the imaging optical system in the main scanning direction; and
fL represents a focal length of the at least one lens in the main scanning direction.
With the above configuration, since a position of the beam spot on the surface to be scanned can be adjusted, the beam spot which should be formed at the center of the scanning line can be actually formed at the center of the scanning line. In steps of adjustment of the at least one lens, since the imaging optical system satisfies the above condition, sensitivity of the adjustment of the at least one lens becomes neither excessively low nor excessively high.
In a particular case, the at least one lens may be movable along a line parallel with the main scanning direction.
In another case, the at least one lens may be rotatable in the plane.
Optionally, the at least one lens may have a first surface which is a light incident side thereof and a second surface opposite to the first surface, at least one of the first surface and the second surface being an aspherical surface.
Further, the at least one lens satisfies a condition:
0.01 less than |[xcex94X1(max)+xcex94X2(max)]/f| less than 0.1
where,
xcex94X1(max) represents an amount of asphericity of the first surface at an outermost position on the first surface in the main scanning direction within an effective diameter of the first lens, the amount of asphericity of the first surface being defined as a difference between a SAG amount of a spherical surface having a curvature corresponding to that of the first surface at an optical reference axis thereof and tangential to the first surface at the optical reference axis and a SAG amount of the first surface;
xcex94X2(max) represents an amount of asphericity of the second surface at an outermost position on the second surface in the main scanning direction within an effective diameter of the second lens, the amount of asphericity of the second surface being defined as a difference between a SAG amount of a spherical surface having a curvature corresponding to that of the second surface at an optical reference axis thereof and tangential to the second surface at the optical reference axis and a SAG amount of the second surface; and
f is a focal length of said imaging optical system in the main scanning direction.
With this configuration, sensitivity of the adjustment of the at least one lens becomes neither excessively low nor excessively high.
Optionally, the imaging optical system may have a scanning lens group being placed adjacent to the deflector, and a compensation lens provided on the side of the surfaces to be scanned with respect to the scanning lens group, the compensation lens compensating for curvature of field.
In this case, the at least one lens is a compensation lens.
Alternatively, the imaging optical system may have a scanning lens group that functions as a scanning lens, all of the plurality of laser beams passing through the scanning lens group.
Optionally, the imaging optical system may have a compensation lens provided for each of the plurality of laser beams, the compensation lens compensating for curvature of field. In this configuration, the scanning lens group is placed adjacent to the deflector, and the at least one lens is a compensation lens provided for each of the plurality of laser beams.
Alternatively, the imaging optical system may have a scanning lens provided for each of the plurality of laser beams, the scanning lens being placed adjacent to the deflector, and a compensation lens provided for each of the plurality of laser beams, the compensation lens compensating for curvature of field.
In this configuration, the at least one lens is a compensation lens provided for each of the plurality of laser beams.
It should be noted that the above-described scanning optical system may be employed in various devices such as a laser beam printer.